Can changing auto transmission fluid in your hhr actually do harm??
 

I want to change the fluid as the car has 125k on it ...but a guy working at a gm wrecking yard told me that changing all the fluid can actually cause the transmission to start slipping and he said change only half the fluid! But that's not easy to do if I have to drop the pan....there is no drain plug right? Catching the fluid and keeping it clean seems tough ..I know the tongue converter holds some fluid maybe it holds enough for a change of fluid in the pan to be 50% of the total fluid?? Any advice would be appreciated

I think he's messing with you! There is no way to change 1/2 the fluid. Ours has been changed twice with no ill effects.

Changing is fine it is back flushing that is bad.
When you drop the pan you only change the fluid that is in the gearbox, you can't drain the torque converter. That is a quart or 2.